The present invention relates generally to the field of data storage. More particularly, the invention relates to data transfer from computer storage media, such as CD-ROM optical storage disks.
Optical storage media in the form of disks are well known as an alternative to magnetic storage media. The disks, commonly known as "compact disks" or "CD's," have a substrate containing pits for storing data. A pit is about 0.5 .mu.m wide, and a disk can hold billions of pits. The pits are aligned in a spiral track similar to the spiral groove in a conventional record. The CD pit track, however, runs from the inside diameter of the disk to the outside.
Information is stored in binary form by the particular arrangement of pits on the plastic substrate. Binary data are encoded in the transitions from the pits and the flat areas ("land") between the pits. The surface is metallized to reflect a laser beam used to read the data.
A specially-designed compact disk player is used to retrieve data stored on a compact disk. The player operates by focusing a laser beam on the reflective metal through a substrate and then detecting the reflected light. The pits increase the optical path of the laser beam by an amount equivalent to one-half wavelength, thereby producing destructive interference when combined with other (non-shifted) reflected beams. Data are read from the disk as a change in intensity of reflected laser light. The reflected light is detected by an optical pick-up or stylus, and the data carried on the beam are converted to digital electrical signals. Further description of the pits and how they function is generally available; see, e.g., Watkinson, The Art of Digital Audio, Focal Press, Chapter 13.
Several different CD formats exist. Perhaps the best known is the CD-Digital Audio (CD-DA) format used for musical recordings. Despite the general association of CD's with music, the media is well suited for other applications. Computer software or other published materials can be stored in a read-only format and delivered as data or video signals in addition to just audio signals.
One such format is CD-ROM which is set forth in Philips/Sony Yellow Book of October 1983. Additional specifications for CD-ROM may be found in the High Sierra specification (from the High Sierra Conference, November 1985) and the more recent ISO (International Standards Organization) 9660 format. While the CD-ROM data format is similar to that of CD-Digital Audio, the disks are not compatible. Other popular formats include the CD-I format (Green Book) which is an interactive, multi-media format and the DVI format which stores full-motion video and audio.
The compact disk is a remarkable storage medium capable of holding over half a gigabyte of storage. However, with this massive storage capability comes the increased difficulty of retrieving the data--metaphorically, locating a kilobyte needle in a gigabyte haystack. Particularly, optical disk systems suffer from slower data access, due in large part to the increased time required for seeking out given data (seek time). As a result, data transfer times for optical disks are substantially longer than for their magnetic counterparts.
Several factors contribute to longer seek times. Part of the problem is due to the design of current optical pickups. For example, moving a head mass of 100 g across a disk in 20 ms has proved difficult. Tracking difficulties also increase the access time. Since the track pitch on an optical disk is so small, the servo must be extremely precise. The required precision is more easily achieved at slower disk rotation. Unlike standard LP records, the rotational rate for CD's is radially dependent, thus the drive motor must also change speed during a seek operation--further adding to seek time. With longer seek times and hence total transfer times, current optical disk systems do not provide access to their data on a real-time basis.
While current mass storage media, such as CD-ROM, provide exceptional storage capability, they cannot be thought of as truly random access media. What is needed is a system which improves data transfer for these media. It is particularly desirable to have a system capable of providing full-color video and stereo sound in real-time. The present invention fulfills this and other needs.